If there's only one transmitter, it's sending the same data to both units, and the only difference is from calibration. Once they are off by even a point, you can't calibrate it the same on both, because being 102 or 108 and entering in 119 will yield different results.
I think we all have a tendency to get too hung up on the exact number. Please remember your fingerstick meter as well as the Dexcom units only show an approximation of what your blood glucose value is. They are not like a BG test at a lab which is very accurate.
That said, the old Dexcom unit has prior data point and fingerstick information that its algorithm can use to refine the BG value it displays. Your new receiver does not have that information to use so it doesn't suprise me at all that the numbers are different eventhough they are both communicating with the same sensor and transmitter.
The glucose present in your interstitial fluid causes a chemical reaction with the sensor wire which in turn generates a small electrical current. It transmits these current values to the receiver. The history of data points and fingerstick values are used by the receivers algorithm to produce a number value (most likely through mathmatical interpolation). If both receivers had the same data points and fingerstick values in them, they would most likely produce the same number, but they don't.
I got the ocean blue one too. That's about all I can add to the conversation. Andy and Phil seem to know the inner workings far more exactly than I and I can't stand math. Good numbers though from where I sit.
Not a significant difference. As the others have said, there are a lot of things that go into calculating the number that shows up. What IS interesting is that they show the same curve, which is pretty much all you can expect from a CGM. And the fact that they are VERY close to what your meter says means they are both functioning pretty much as well as could be expected. Yay for Dexcom!
Phil has it down well. Even if you pulled one tube of blood in a lab and ran the same lab test twice on the same sample, you can get values of ±2%. So for a real bG of 100mg/dL, a lab can get 98-102mg/dL. The same sample on a finger stick bG is ±15% at best, so 85 to 115mg/dL. If I am reading the Dexcom G4 information correctly, it is ±15-20%.
Bottom line, all of these are tools. We all need to learn 1)what they are, 2)how they work, and 3)how to use them to our advantage.
You mentioned that you "received a new Dexcom to replace my broken rail." Not sure what you meant by "rail" but there's another discussion I've found about the sliding cover for the USB port having problems -- I just posted a photo of my SECOND broken sliding cover "rail" -- is this the problem you had?
I'm trying to get an idea how common the breakage problem is...